1. Field of the Invention
The present invention relates to a zoom lens having a diffractive optical element and, more particularly, to a zoom lens of high range, while still assuring maintenance of high image quality, suited to photographic cameras, video cameras, or digital still cameras.
2. Description of Related Art
As the photographic film or CCD is ever more improved by decreasing the size of silver-halide grains or by increasing the number of pixels, there is a growing demand that the zoom lens for silver-halide film camera, video camera or digital still camera have, despite the high range, to provide pictures of ever higher quality. Recently, the production techniques of aspherical lenses and the design efforts have advanced so greatly that all the aberrations that determine the lens performance become easier to correct even in a lesser number of constituent lenses.
Concerning chromatic aberrations, however, their correction depends on the dispersion differences among colors for the glass materials of the constituent lenses and the combination of positive and negative lenses in the achromatic condition. Therefore, any improvements of the achromatism cannot be expected from the use of the aspherical lens.
Meanwhile, there has been known another method of suppressing the produced amount of chromatic aberrations to a minimum for good stability of image aberrations. So, in recent years, proposals for applying diffractive optical elements to the photographic optical system are made in, for example, Japanese Laid-Open Patent Applications No. Hei 4-213421 (U.S. Pat. No. 5,044,706) and No. Hei 6-324262. These conventional examples are concerned with mono-focal lenses to which the diffractive optical element is applied. Although chromatic aberrations have been mentioned there, what is peculiar to the zoom lens, that is, the variation with zooming of aberrations is not considered or described. Hence, the application of the diffractive optical element to the zoom lens is not suggested there.
Concerning the zoom lens, there is a description about adapting the application of the diffractive optical element to the zoom lens in U.S. Pat. No. 5,268,790. In this example, what has been proposed is to arrange the diffractive optical element in the second lens unit as the main variator or in the third lens unit as the compensator. As for the first lens unit, the design is made as usual. In this form, the chromatic aberrations the first lens unit produces are left as they were. In some cases, therefore, the chromatic aberrations of the entire lens system unduly worsen at certain zooming positions.